The present invention relates to speech coding and more particularly to improvements in extraction and coding of sound source or excitation information directed to reduction of the number of processing steps.
As a-coding system suitable for compressing speech information to 8 to 16 k bps, there is available a thinned-out residual (TOR) method proposed by the present applicants. See Japanese Patent Application No. 59-5583 or Akira Ichikawa et al "A SPEECH CODING METHOD USING THINNED-OUT RESIDUAL", ICASSP 85, 1985. The TOR method intends to improve the quality of coded speech by making more precise the excitation of a linear predictive coding (LPC) vocoder system such as a partial autocorrelation (PARCOR) system. In the TOR method, to compress the information, pulses of less importance from the standpoint of quality are thinned out or decimated from a predictive residual pulse train as a predictive error resulting from the LPC analysis effected in a unit of a frame of a voice or speech data signal. The TOR concludes that residual pulses of smaller amplitude are permitted to be decimated in preference to those of larger amplitude and it does not require any error evaluation computation for decimation, thus succeeding in reducing the number of processing steps to some extent.
However, to effect the decimation of the small amplitude residual pulses (in other words, to effect the extraction of large amplitude pulses), the TOR method requires a process for sorting a number of residual pulses in one frame (amounting to 160 pulses where the sampling rate is 8 KHz and the frame period is 20 mS) and faces difficulties in making the system compact.